The present invention belongs to a technical field of illumination, and particularly to a surface illuminant device and a prism sheet used therefore
In a liquid crystal display device (liquid crystal display) used to constitute a display screen of a note type personal computer, back illumination is used to make the display bright and easily viewable. A so-called edge light type surface illuminant device in which a linear light source is disposed along the end face of a plate-shaped light guide to emit illumination light from one principal surface (obverse surface) of the light guide is used in order to irradiate illumination light to the overall area of the display screen as uniformly as possible and also make the overall construction of the display device containing the illuminant light source as thin as possible. Such an edge light type surface illuminant device as described above is described in JP(U)-3-69184, JP(B)-7-27136, JP(B)-7-27137, etc.
In the surface illuminant device or planar light source device as described above, light which is emitted from the linear light source and incident into the light guide is transmitted to the end face confronting the incident end face in the light guide while repetitively reflected from the obverse surface thereof and the back surface thereof which is formed substantially in parallel to the obverse surface or obliquely to the obverse surface. A part of the light is emitted from the light emission face of the light guide by a light emitting structure formed in the light guide in course of the light transmission. The emission light has a directivity in a direction different from a viewing direction. Therefore, a prism sheet or the like is used in order to emit light while changing the directivity of the light to a required direction With respect to the prism sheet, as disclosed in JP(U)-3-69184, the prism face thereof is frequently disposed so as to confront the opposite side to the light guide, and prism sheets are usable to be superposed on each other so that the directions of the prism ridge lines are perpendicular to each other. Furthermore, there has been proposed a surface illuminant device in which the prim face of the prism sheet is disposed in such a manner as to face the light guide side as disclosed in JP(B)-7-27136 and JP(B)-7-27137.
Meanwhile, the display screen of the note type personal computer has been recently required to be increased in size, and in order to satisfy this requirement, the surface illuminant device has been also required to be increased in area. Even when the area is increased as described above, it is required to generate illumination light which is as uniform and high in intensity as possible over the whole surface. Particularly, with respect to a note type personal computer or liquid crystal monitor, it is viewed from a short distance of about 30 to 40 cm, and thus the angle at which light directing to the eyes of a viewer intersects to the normal line of the screen is greatly different between the center portion and the peripheral portion of the screen, which causes non-uniformity of visibility.
JP(A)-7-318729 discloses a surface illuminant device in which such a linear Fresnel lens sheet that the intersecting angle of the long side of each elongated prism to the sheet surface is increased as it is far away from the position beneath the point of view on the cross section perpendicular to the elongated prisms is disposed on the light emission face of the light guide so that the lens face serves as the light emission surface, thereby converging the emission light to the point-of-view position or eyes position.
In such a surface illuminant device, the emission light can be converged to the point-of-view position However, the lens sheet is disposed so that the lens face serves as the light emission surface and the emission light is focused by the refraction action of the long side of the prism, so that the light using efficiency is low and thus no surface illuminant device having sufficiently high brightness can be achieved. Furthermore, since the emission light is focused by the refraction action of the prism long side, it is required to use a diffusion sheet or the like so as to deflect the emission light from the light guide in a direction of about 0 to 30 degrees with respect to the normal line of the sheet surface. As described above, the emission light from the light guide having high directivity cannot be directly focused in the point-of-view position, resulting in reduction of the brightness. Still furthermore, since the emission light is focused by the refraction action of only the prism long side, light which is reflected and returned from one end face of the light guide when a linear light source is disposed at the other end face of the light guide cannot be focused in a prescribed position, and also light which comes from a light source disposed at the side of the prism long side when linear light sources are equipped at both the end faces of the light guide cannot be focused in a prescribed position, and thus the light using efficiency is reduced.
Therefore, an object of the present invention is to provide an edge light type surface illuminant device for illumination of a screen which is viewed from a relatively short distance, which can be easily increased in area without occurrence of non-uniformity of visibility.
That is, according to the present invention, there is provided a surface illuminant device or planar light source device having a plate-shaped light guide in which one principal surface serves as a light emission face and at least one end face serves as a light incident face, a light source disposed so as to extend along the light incident face of the light guide and a light travel direction converting prism sheet disposed on the light emission face of the light guide, characterized in that an incident surface of the prism sheet is disposed so as to face the light emission face of the light guide, plural elongated prisms are arranged in parallel to one another on the incident surface of the prism sheet so as to extend substantially in parallel to the light incident face of the light guide, the light guide has a light emitting structure with which light introduced into the light guide from the light source through the light incident face is emitted from the light emission face, and there is provided light focusing means for acting on light passed through the light guide and the light travel direction converting prism sheet so that the light is emitted from the prism sheet under a focused state.
Furthermore, according to the present invention, there is provided a prism sheet for use in a surface illuminant device as described above, characterized in that plural elongated prisms are arranged in parallel to one another on one surface thereof, and the apex angles xcex1 of the elongated prisms and the variation rate xcex3 of the inclination angles xcex2 of the elongated prisms to the normal direction of the prism sheet in a direction perpendicular to the extending direction of each elongated prism satisfy the following conditions:
55 degreesxe2x89xa6xcex1xe2x89xa675 degrees
0.2 degree/cmxe2x89xa6xcex3xe2x89xa61.5 degree/cm.
Still furthermore, according to the present invention, there is provided a surface illuminant device or planar light source device having a plate-shaped light guide in which one principal surface serves as a light emission face and at least one end face serves as a light incident face, a light source disposed so as to extend along the light incident face of the light guide and a light travel direction converting prism sheet disposed on the light emission face of the light guide, characterized in that an incident surface of the prism sheet is disposed so as to face the light emission face of the light guide, plural elongated prisms are arranged in parallel to one another on an emission surface at the opposite side to the incident surface of the prism sheet so as to extend substantially in parallel to the light incident face of the light guide, the incident surface or emission surface of the prism sheet is curved in a convex or concave shape in the direction perpendicular to the light incident face of the light guide, and said light guide has a light emitting structure for emitting from the light emission face light which is introduced into the light guide from the light source through the light incident face.